The term means "rotation of duties"[1][2] and first appears in the Tactica of Leo VI the Wise in the early 10th century for a generic body of troops.
[5] From the late 11th century, as evidenced in the writings of Michael Attaleiates, the term also began to be used in a more specific sense for the troops of the imperial bodyguard.
[3] By the late 13th century, the term had largely replaced the earlier tagma in colloquial and technical (although not entirely in literary) usage to designate any standing regiment.
His uniform comprised a skiadion hat decorated with gold wire, a kaftan-like kabbadion in silk "as it is commonly used", a velvet-covered skaranikon with a red tassel on top, and a baton of office of plain smooth wood.
With the gradual fall of Asia Minor to the Turks during the late 13th and early 14th centuries, the "imperial allagia" finally disappeared.
[12] As Mark Bartusis comments on the various attempts to explain their role, "at the one extreme the megala allagia were the central element in the late Byzantine army; every soldier who lived in the provinces and who had a military obligation [...] was a megaloallagitēs...", meaning that they represented a universal military organization involved in the recruitment and maintenance of all provincial forces, from which only the imperial guards and the personal retinues of local governors must be excluded.
On the other extreme, the megala allagia may have been only a partial aspect of the late Byzantine military system, confined only to some provinces and from which foreign mercenaries were probably excluded.